Photodynamic Therapy (PDT) is presently undergoing extensive basic, preclinical and clinical testing for the diagnosis and treatment of cancer. Regulatory approval for the first PDT drug or photosensitizer, Photofrin, is expected in 1993. Concurrent with this is the introduction of a number of second generation photosensitizers into clinical trials in late 1992 and early 1993. These second generation photosensitizers all have red wavelength activation peaks in the 660nm to 700nm range, which corresponds to the operating range of high power red laser diodes, InGaAlP, presently under development by a number of laser diode manufactures. If these laser diodes can be efficiently coupled into larger single optical fibers, be wavelength stabilized, and provided in a small overall package, they could significantly impact the clinical application of these new PDT drugs by eliminating the need for $100,000 plus laser systems presently required. It is the aim of this Phase-I project to develop and test a Prototype high power 670nm laser diode system for the use the second generation photosensitizer, SnET2. This prototype system will include fiber optic coupling of a high power phased array diode into a 400 micron core fiber, along with wavelength selection through temperature stabilization if the laser diode. Predicted performance is over 2.0 watt of power at 670nm with 3.0nm bandwidth (FWHM). The system will be tested in a rat tumor model in conjunction with the Medical College of Ohio. In Phase II a higher power clinical prototype system will be developed with output in the 3.0 watt - 4.0 watt range, and tested with SnET2 and other second generation photosensitizers in the 660nm to 700nm range. If the system can be developed into high power (3 to 4 watts), low cost (less than $30K) and easy to use device, it could significantly increase the potential of PDT by opening the treatment up to small hospitals, outpatient surgery units and doctor's offices.